TEM characterization of Cu self-annealing and direct proof of pinhole formation mechanism in a Cu film

Pinhole formation in electroplated Cu has been a critical reliability issue in developing through-/blind-hole (TH/BH) metallization with a thin surface-Cu feature. A mechanism of the pinhole formation has been recently proposed in a previous study, which hypothesized that crystallographic defects (or organic impurities) might be incorporated into channels within the as-deposited Cu platings. The existence of the channels accompanying with numerous material defects would facilitate the pinhole formation in the chemical etching process. The focus of this study was to validate this hypothesis and to establish the mitigation to pinhole formation through high-resolution transmission electron microscopy (TEM) in combination with energy dispersive X-ray spectroscopy (EDS). We investigated the Cu microstructure and the pinhole structure in different self-annealing stages, to establish the isothermal grain transformation kinetics of Cu and the correlation between Cu microstructure evolution and pinhole formation behavior. The mitigation can greatly improve the material characteristics of electroplated Cu in the TH/BH metallization.